Xeroprinting, as described by Schaffert, R.M., in Electrophotography (2nd Edit., Focal Press, 1980) at pp. 209-210, is the name given to an electrostatic printing process designed to print many copies of an original from a master plate or cylinder. The printing master or plate consists of a metal sheet or cylinder on which is imprinted an image in the form of a thin insulating coating. The image areas consist of insulating material and the background areas are bare metal.
When the master plate is electrically charged by passing it under a corona charging unit, the charges deposited on the insulated image remain on the image surface, and those deposited on the bare metal areas are conducted away to ground. This forms an electrostatic image corresponding to the insulated areas. The image is then developed with toner which is transferred and fused to a receiver sheet.
U.S. Pat. No. 4,914,477, issued to Young et al discloses an apparatus which may be used to create xeroprinting masters. An electrophotographic web, having a photoconductive layer, a conductive layer and a support layer, is charged at a charging station and exposed at an exposing station to create an electrostatic image on the web. This image is toned at a toning station with electrically insulating toner to create a toner image. The toner image is then transferred and fused to a receiver sheet having an outer conductive layer. The result is a xeroprinting master which may be used in a xeroprinting apparatus. Color separation xeroprinting masters may be created by exposing the web according to color separation image data and developing the resultant electrostatic images with the insulating toner.
The Young apparatus is intended to be part of a system including separate master-making and master-using machines. The master-making machine would be slow and electronically controlled, producing very high quality xeroprinting masters. The master-using machine would be fast and heavy-duty. This system allows masters for one printer job to be made while another job is being printed.
U.S. Pat. No. 2,576,047 to Schaffert discloses a xeroprinting apparatus which is capable of utilizing the xeroprinting master created by the Young et al apparatus. A rotatably mounted drum has a xeroprinting master (called a "plate" in Schaffert) flexed around and attached to the drum. The xeroprinting master is held to the drum by conventional means and the conductive layer of the master is electrically connected to the drum which is ground. A uniform charge is applied to the master. This charge remians only on the insulating toner. The resultant electrostatic image is developed with toner to create a visible image. This visible image is transferred and fused to a receiver sheet.
The apparatus disclosed in Schaffert may be adapted to produce multicolor prints from a multicolor original. Four color separation xeroprinting masters, representing red, green, blue and black light images of a multicolor original, are each secured to a drum. Each master is charged, and the resultant electrostatic image is developed with a certain color toner. The red master is developed with cyan toner, the green master is developed with magenta toner, blue master is developed with yellow toner and the black master is developed with black toner. The four toned images are transferred in superimposed registration with one another to a receiver sheet to form a multicolor print thereon.
A problem with this form of xeroprinting is that if the xeroprinting master is secured to the drum askew, any copies made from the master will also have images which are askew from the edges of the copy. Additionally, the master sheet may be deformed by overtensioning it when the master is secured to the drum. This will cause lines of image information to spread apart as well as cause the image information in individual lines to crowd together. This problem is compounded in a multicolor xeroprinter. In the prior art, a front and side edge of each master is used to register each master in the xeroprinting apparatus. That registration must be accurate enough to obtain a final multicolor print in which the colors are tightly in register. If each of the color separation masters are not secured to their respective drums in exactly the same orientation, color misregistration will result. Correcting for skew color misregistration will involve the very difficult task of realigning the master sheets on the drum because skew misregistration cannot be corrected by reorienting the drums themselves.